Reducing noise in interior Scenes caused by many lights using Cycles
I guess, that you know most of the common tricks. But here are some tricks that I often use:


Make a hole into the building outside of the cameras view of field
Increase the resolution, decrease the sample rate and resize the image afterwards. This will make the image softer, but if you double the image size, and render with a low sample rate and resize the image afterwards, the noise is evened out (I was able to render an interior architectural scene with only 5 samples)
Enable a small amount of global illumination and increase the shadows afterwards with the Compositor